Methods and systems for exhaust gas mixing

ABSTRACT

A method of operating an engine system is described in which a back-pressure valve of a low pressure EGR system can be modulated at a lower frequency during an EGR operation, and at a higher frequency when additional exhaust gas sensing is requested. Back-pressure valve modulation can therefore be used to promote exhaust gas mixing while providing EGR. Exhaust gas constituents in the mixed exhaust gas can then be sensed with higher accuracy by downstream exhaust gas sensors.

FIELD

The present application relates to methods and systems for mixingexhaust gas constituents in an engine exhaust system.

BACKGROUND AND SUMMARY

In an effort to meet stringent federal government emissions standards,engine systems may be configured with exhaust gas recirculation (EGR)systems wherein at least a portion of the exhaust gas is recirculated tothe engine intake. Additionally various sensors may be included in theengine exhaust manifold to estimate tailpipe emissions.

One example of such an EGR system is illustrated by Lutz et al. in US2008/0223038 A1. Herein, a low pressure EGR (LP-EGR) system isconfigured to recirculate exhaust gas from downstream of a particulatefilter to the engine intake. An amount of EGR is adjusted via an LP-EGRvalve positioned in the LP-EGR bypass. Additional EGR adjustments may beperformed via an exhaust gas back-pressure valve positioned downstreamof the EGR bypass. Further, various sensors (for example, a NOx sensor,an air-fuel ratio sensor, and a particulate matter sensor) may beincluded in the engine exhaust manifold to estimate tailpipe emissions.

However, the inventors herein have recognized a potential issue withsuch systems. Exhaust gas constituents may not be uniformly distributed.Consequently, there may be a discrepancy between the concentration of anexhaust gas constituent as estimated by a sensor in the exhaustmanifold, and the concentration of the constituent in the bulk exhaustgas. Thus, the accuracy of the sensor may be degraded leading todegraded engine emissions. While EGR valve adjustments may provide someexhaust gas mixing, even small EGR valve modulations may have largeeffects on an amount of EGR provided, thereby degrading the desired EGRcontrol. Fixed exhaust mixers may alternatively be included in theengine exhaust to improve exhaust gas mixing, however, such mixers mayadd substantial component costs.

Thus, in one example, the above mentioned issue may be addressed byimproving exhaust gas mixing through a method of operating an engineincluding an EGR system coupled between an engine intake and an engineexhaust. The method may comprise, adjusting a back-pressure valve of theEGR system to provide a desired EGR amount, the adjusting below a firstfrequency, and selectively modulating the back-pressure valve above thefirst frequency to promote exhaust gas mixing, while maintaining thedesired EGR amount.

In one example, an engine may include an exhaust gas recirculation (EGR)loop coupling the engine exhaust manifold to the intake manifold. Thelow pressure EGR loop may recirculate at least some exhaust gas fromdownstream of an emission control device in the exhaust manifold, toupstream of an intake throttle in the intake manifold. An amount ofexhaust gas recirculated to the intake may be adjusted by ways of an EGRvalve included in the EGR loop. An additional back-pressure valve,positioned in the exhaust manifold, downstream of the EGR loop and theemission control device, and upstream of exhaust emission sensors, maybe configured to provide additional back-pressure to drive exhaust gasflow into the EGR loop. During selected engine operating conditions, EGRmay be provided by adjusting the EGR valve and the back-pressure valve.For example, an amount of EGR may be increased by opening the EGR valve,and partially closing the back-pressure valve. The back-pressure valvemay be adjusted at a lower frequency (or below a predefined frequency)to provide the desired amount of EGR. Herein, by recirculating heatedexhaust gas to the engine intake, engine exhaust emissions and fueleconomy may be improved.

In one example, the emission control device may include a dieselparticulate filter (DPF). Herein, to improve filter operations, andconsequently exhaust emissions, exhaust NOx and/or particulate matter(PM) levels may be periodically sensed by one or more exhaust sensorspositioned downstream of the DPF and the back-pressure valve. Duringselected conditions when sensing is requested, (for example, whensensing is requested to determine whether the filter needs to beregenerated, or when sensing is requested for filter leak detectionroutines), the back-pressure valve may be modulated (for example,sinusoidally pulsed) to create a local turbulence that mixes the exhaustgas. Specifically, the back-pressure valve may be selectively modulatedat a higher frequency (or above the predefined frequency) such thatexhaust gas mixing is achieved without affecting the average EGR flow.For example, the back-pressure valve may be adjusted with a firstfrequency during a first EGR condition when no sensing is requested.Then, during a second EGR condition, when sensing is requested, inaddition to the adjustment, the back-pressure valve may be modulatedwith a second, higher frequency component.

The modulation frequency, during sensing, may be determined based onengine operating conditions, and may be selected so as to maintain therequested amount of EGR. For example, the frequency may be selected suchthat the average back-pressure valve position is maintained at theposition desired for the requested amount of EGR. In one example,determining the modulation frequency based on engine operatingconditions may include increasing the frequency of modulation as anengine speed increases. Additionally, the amplitude of modulation may beadjusted based on engine operating conditions. For example, theadjustment may include decreasing the amplitude of the modulation asengine valve timing approaches valve opening or valve closing limits.

In still another embodiment, the EGR valve may be selectively modulatedin addition to the back-pressure valve modulation during sensing. TheEGR valve may be an HP-EGR valve and/or an LP-EGR valve. Herein, the EGRvalve modulation may be adjusted in coordination with the back-pressurevalve modulation such that the requested amount of EGR is maintained.For example, the frequency (and amplitude) of EGR valve modulation maybe adjusted to match the frequency of the back-pressure valvemodulation, while a phase of the EGR valve modulation may be adjusted tooff-set the back-pressure valve modulation.

In this way, by applying a periodic motion to the back-pressure valve, alocal stirring of exhaust gases may be achieved that may improve exhaustgas mixing without affecting EGR flow. By improving the mixing ofexhaust gases, the sensing accuracy of exhaust gas sensors may beimproved. Additionally, by using components already in use in EGRsystems to actively mix the exhaust gas, the need for dedicated mixers,such as fixed exhaust gas mixers, may be reduced.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of an engine and an associatedexhaust gas recirculation system.

FIGS. 2-3 show high level flow charts illustrating routines that may beimplemented for improving exhaust gas mixing.

FIG. 4 shows example exhaust gas mixing operations according to thepresent disclosure.

DETAILED DESCRIPTION

The following description relates to systems and methods for improvingexhaust gas mixing in an engine exhaust. As shown in FIG. 1, an engineconfigured with a low-pressure and a high-pressure exhaust gasrecirculation (EGR) system may include an EGR valve in each EGR loop foradjusting an amount of exhaust gas recirculated to the engine intake. Aback-pressure valve positioned downstream of the LP-EGR loop may beadjusted to further assist in diverting exhaust gas through the EGRloops. As illustrated in FIG. 4, during conditions when exhaust gassensing is requested, for example to determine filter regeneration orfilter leak detection, the back-pressure valve may be additionallymodulated at a higher frequency to promote exhaust gas mixing whilemaintaining the EGR operation. In response to the request for sensing,an engine controller may be configured to perform a control routine,such as the routines of FIGS. 2-3, to select the higher frequency of theback-pressure valve modulation based on engine operating conditions. Thecontroller may also adjust the amplitude of the back-pressure valvemodulation based on the operating conditions. One or more of the LP-EGRvalve and the HP-EGR valve may also be modulated by the controller,based on the back-pressure valve modulation, to maintain the desired EGRrate. In this way, exhaust gas mixing may be achieved without affectingthe EGR flow. The mixed exhaust gas may then be sensed accurately byexhaust gas sensors positioned downstream of the back-pressure valve. Byimproving exhaust gas mixing without affecting EGR, exhaust emissionsmay be accurately sensed and engine emission control may be improved.

FIG. 1 shows a schematic depiction of a vehicle system 6. The vehiclesystem 6 includes an engine system 8, including engine 10, coupled toemission control system 22. Engine 10 includes a plurality of cylinders30. Engine 10 also includes an intake 23 and an exhaust 25. Intake 23includes a throttle 62 fluidly coupled to the engine intake manifold 44via intake passage 42. Exhaust 25 includes an exhaust manifold 48leading to an exhaust passage 45 that routes exhaust gas to theatmosphere via tailpipe 35.

Engine 10 may further include a boosting device, such as turbocharger50. Turbocharger 50 may include a compressor 52 arranged along intakepassage 42. The compressor 52 may be at least partially driven byturbine 54, arranged along exhaust passage 45, via shaft 56. The amountof boost provided by the turbocharger may be varied by an enginecontroller. An optional charge after-cooler (not shown) may be includeddownstream of compressor 52 in the intake passage to reduce thetemperature of the intake air compressed by the turbocharger.

Emission control system 22, coupled to exhaust passage 45, may includeone or more emission control devices 70 mounted in a close-coupledposition in the exhaust. One or more emission control devices mayinclude particulate filter 76, SCR catalyst, three-way catalyst, leanNOx trap, oxidation catalyst, etc. The emission control devices may bepositioned upstream and/or downstream (as depicted) of turbine 54 inexhaust passage 45. In an alternate embodiment, particulate filter 76may be positioned upstream of the turbocharger turbine 54 while a NOxtrap or SCR catalyst is positioned downstream of the turbine. Theparticulate filter 76 may be, for example, an uncoated dieselparticulate filter. Alternatively, the particulate filter 76 may includea catalytic washcoat. Catalytic washcoats used may include, for example,palladium, a hydrocarbon adsorbent (such as activated carbon orzeolite), an SCR catalyst, a HC adsorbent-SCR catalyst combination, etc.

Engine 10 may further include one or more exhaust gas recirculation(EGR) passages for recirculating at least a portion of exhaust gas fromexhaust passage 45 to intake passage 42. For example, the engine mayinclude a low-pressure EGR (LP-EGR) system 72 configured to divert aportion of exhaust gas from downstream of turbine 54 and emissioncontrol device 70 to the engine intake upstream of compressor 52 andthrottle 62 via LP-EGR loop 73. LP-EGR system 72 may be operated duringconditions such as in the presence of turbocharger boost and/or whenexhaust gas temperature is above a threshold. LP-EGR loop 73 may furtherinclude an LP-EGR cooler 74 to lower the temperature of exhaust gasbeing recirculated into the engine intake. The engine may also include ahigh-pressure EGR (HP-EGR) system 92 configured to divert a portion ofexhaust gas from upstream of turbine 54 to the engine intake downstreamof compressor 52 and upstream of throttle 62 via HP-EGR loop 93. HP-EGRsystem 92 may be operated during conditions such as in the absence ofturbocharger boost. HP-EGR loop 93 may further include an HP-EGR cooler94 to lower the temperature of exhaust gas being recirculated into theengine intake.

An amount and/or rate of exhaust gas recirculation through LP-EGR loop73 may be varied by controller 12 via LP-EGR valve 39. One or moreexhaust gas sensors, such as EGR sensor 75, may be positioned withinLP-EGR loop 73 to provide an indication of one or more of a pressure,temperature, and air-fuel ratio of exhaust gas recirculated throughLP-EGR system 72. Similarly, an amount and/or rate of exhaust gasrecirculation through HP-EGR loop 93 may be varied by controller 12 viaHP-EGR valve 99. One or more exhaust gas sensors, such as EGR sensor 95,may be positioned within HP-EGR loop 93 to provide an indication of oneor more of a pressure, temperature, and air-fuel ratio of exhaust gasrecirculated through HP-EGR system 92.

An exhaust back-pressure valve 80 (herein also referred to as an exhaustthrottle) may be included downstream of LP-EGR loop 73 to createsufficient back-pressure to drive exhaust gas through the LP-EGR loop.During conditions when EGR is requested, LP-EGR valve 39 andback-pressure valve 80 may be adjusted to divert an amount of exhaustgas through the EGR loop to provide the desired amount of EGR. An enginecontroller may adjust back-pressure valve 80 below a first frequency toprovide the EGR. Alternatively, HP-EGR valve 99 may be adjusted todivert an amount of exhaust gas through the EGR loop to provide thedesired amount of EGR. By recirculating heated exhaust gas to the engineintake, improved emission and fuel economy benefits may be achieved.

Engine 10 may be controlled at least partially by a control system 14including controller 12 and by input from a vehicle operator via aninput device (not shown). Control system 14 is shown receivinginformation from a plurality of sensors 16 (various examples of whichare described herein) and sending control signals to a plurality ofactuators 81. As one example, sensors 16 may include exhaust gas sensor126 located upstream of the emission control device, exhaust temperaturesensor 128 and exhaust pressure sensor 129 located downstream of theemission control system in tailpipe 35, and EGR sensors 75, 95 locatedin LP-EGR loop 73 and HP-EGR loop 93. Various exhaust gas sensors mayalso be included in exhaust passage 45 downstream of exhaustback-pressure valve 80, such as a particulate matter (PM) sensor 82, NOxsensor 84, oxygen sensor (not shown), ammonia sensor (not shown),hydrocarbon sensor (not shown), etc. Other sensors such as additionalpressure, temperature, air/fuel ratio and composition sensors may becoupled to various locations in the vehicle system 6. As anotherexample, actuators 81 may include fuel injector 66, EGR valve 39,throttle 62, and back-pressure valve 80. Other actuators, such as avariety of additional valves and throttles, may be coupled to variouslocations in vehicle system 6. Controller 12 may receive input data fromthe various sensors, process the input data, and trigger the actuatorsin response to the processed input data based on instruction or codeprogrammed therein corresponding to one or more routines. An examplecontrol routine is described herein with regard to FIG. 2.

During select EGR conditions, additional exhaust gas sensing may berequested. For example, exhaust gas sensing may be requested during leakdetection routines, or to determine if particulate filter 76 needsregeneration. As such, insufficient exhaust gas mixing in the tailpipemay lead to inaccurate exhaust gas sensing by the exhaust gas sensors.Consequently, emission control may be degraded. In one example, when theengine is operating at low engine speed but with a high load, there maybe insufficient exhaust gas mixing. Thus, as further elaborated in FIGS.2-3, in response to a request for sensing, an engine controller mayselectively modulate back-pressure valve 80 in addition to the adjustingof the back-pressure valve to provide the requested EGR. For example,the engine controller may modulate back-pressure valve 80 above thefirst frequency to promote exhaust gas mixing while maintaining the EGRflow. Herein, the contraction and expansion of exhaust flow across thevalve causes a local turbulence that improves exhaust gas mixing. Themodulation frequency and/or amplitude may be selected based on engineoperating conditions. By adjusting a back-pressure valve to provide EGRand by further modulating the back-pressure valve in response to arequest for sensing, exhaust gas mixing may be achieved whilemaintaining EGR flow characteristics. Additionally, by using EGRcomponents to actively mix the exhaust gas, the need for dedicatedexhaust gas mixers may be reduced, and component reduction benefits maybe achieved.

In another example, the back-pressure valve may be modulated to createturbulence for mixing the exhaust gas to facilitate downstream exhaustgas sensing, while the HP-EGR valve and/or LP-EGR valve iscorrespondingly modulated to maintain the EGR flow rate. As elaboratedwith reference to FIGS. 2-4, by modulating the EGR valve (LP-EGR valveor HP-EGR valve) based on the modulation of the back-pressure valve, EGRflow may be maintained while enabling sufficient exhaust gas mixing.

Now turning to FIG. 2, an example routine 200 is depicted for adjustinga back-pressure valve coupled to an EGR system to provide a desiredamount of EGR and to promote exhaust gas mixing in response to a requestfor exhaust gas sensing.

At 202, engine operating conditions may be estimated and/or measured.These may include, for example, engine speed, driver-requested torque,engine temperature, catalyst temperature, etc. At 204, a desired amountof EGR may be determined based on the estimated operating conditions. At206, the EGR valve may be adjusted based on the requested amount of EGR.For example, the EGR valve may be opened to increase the amount of EGRprovided to the engine intake. At 208, the exhaust back-pressure valvemay also be adjusted based on the engine operating conditions to providethe requested amount of EGR. For example, when the EGR valve is opened,the back-pressure valve may be partially closed to provide theback-pressure required to divert exhaust gas into the EGR loop. Theback-pressure valve adjustment may be performed below a first frequency.At 210, it may be determined if a request for sensing has been received.In one example, a request for exhaust gas sensing may be received duringa leak detection routine. In another example, a request for exhaust gassensing may be received after a threshold duration of engine operationhas elapsed to determine if filter regeneration is required.

If no request for sensing is received at 210, the routine may end.However, if a request for sensing is received, then at 212, the enginecontroller may additionally modulate the back-pressure valve (on top ofthe adjustment to provide the desired EGR flow, for example), bymodulating the back-pressure valve at a frequency above the firstfrequency to promote exhaust gas mixing while maintaining the averageEGR flow (averaged over the modulation period) at the desired EGRamount. The modulation frequency may be adjusted based on the estimatedengine operating conditions, including engine speed. The adjustment mayinclude, for example, increasing the modulation frequency as enginespeed increases. As such, the frequency of the modulation may be keptwithin the bandwidth of the back-pressure valve. Additionally, anamplitude of the modulation may be adjusted based on the engineoperating conditions. For example, the amplitude may be decreased as thevalve opening of the back-pressure valve approaches valve fully opened(e.g., an open stop) or fully closed (e.g., a close stop).

While one example includes sinusoidal modulation of the back-pressurevalve on top of the adjustments providing the average desired EGR flow,various other modulation schemes may be used. For example, modulationincluding a plurality of sinusoidal components, each one at a frequencyabove the first frequency, may be used, where the various frequencies ofthe sinusoidal components may be selected based on operating conditionssuch as exhaust gas flow rate, engine speed, exhaust pressure, exhausttemperature, etc.

As further elaborated with reference to FIG. 3, the engine controllermay optionally maintain the amount of EGR by also adding modulation tothe HP-EGR valve and/or LP-EGR valve, wherein the EGR valve modulationmay be based on the modulation of the back-pressure valve. This mayinclude, adjusting a frequency and amplitude of the EGR valve modulationto match the frequency of the back-pressure valve modulation, whileadjusting a phase of the EGR valve modulation relative to the modulationof the back-pressure valve to offset the effect of the back-pressurevalve modulation on instantaneous EGR flow into the engine. For example,the EGR valve modulation may be at a common frequency, but out of phasewith, the back-pressure valve modulation to effectively cancel out themodulation's effect on the instantaneous EGR flow.

By modulating the back-pressure valve, a local stiffing of exhaust gasesmay be achieved which may promote exhaust gas mixing, when requested. At214, the mixed exhaust gases may be sensed by the exhaust gas sensorsdownstream of the back-pressure valve. By mixing the exhaust gasesbefore and upstream of the sensors, the sensing accuracy of the sensorsmay be improved, thereby improving emission control. Additionally, byselectively applying the modulation, it is possible to conserve energywhen sufficient exhaust mixing occurs without modulation. For example,FIG. 2 shows providing valve modulation during sensing of one or more ofsensors 82 or 84, where modulation is not carried out when there is nosensing. However, in another example, the sensors may be monitoredduring engine operation (and used as a basis for adjusting engineoperation, filter regeneration, and the like) during both modulation ofthe valve, and during adjustment of the valve without the addedmodulation. For example, conditions at higher engine speeds and higherloads may generate sufficient exhaust gas mixing, such that nomodulation is needed, and thus no modulation may be provided under theseselected conditions. Further still, whether or not the modulation isadded to the back-pressure valve may be based on a position of theback-pressure valve, in that more open positions generate lessturbulence, whereas more closed positions generate more turbulence. Assuch, in one example, during a first condition (such as a firstback-pressure valve position being less than a threshold), an operatingcondition (e.g., initiation of filter regeneration) may be based onsensor readings of a sensor downstream of the back-pressure valve (suchas from sensor 82) without additional sinusoidal modulation of theback-pressure valve; and, during a second condition (such as a secondback-pressure valve position being greater than a threshold), theoperating condition may be based on sensor readings of the downstreamsensor with additional sinusoidal modulation of the back-pressure valveto increase turbulence and thus accuracy of the sensor reading.

Turning now to FIG. 3, an example routine 300 is described formaintaining an EGR flow during the back-pressure valve modulation. At302, the back-pressure valve may be modulated above a first frequency toprovide exhaust gas mixing. At 304, it may be determined whether it isdesired to maintain an EGR flow rate with an EGR valve. Herein, the EGRvalve may be an LP-EGR valve of the LP-EGR system and/or an HP-EGR valveof the HP-EGR system. If it is not desired to maintain the EGR flow ratewith an EGR valve, then at 306, the modulation amplitude and frequencyof back-pressure valve modulation may be adjusted based on the engineoperating conditions so as to maintain the EGR flow rate while providingthe requested turbulence mixing. In one example, EGR valve modulationmay not be requested due to the EGR valve motion being limited orrestricted. For example, the position of the EGR valve may be too closeto a fully open or fully closed position such that modulation may not bepossible.

In comparison, if it is desired to maintain the EGR flow rate with anEGR valve, for example, when the EGR valve's motion is not limited orrestricted, then at 308, one or more of the HP-EGR valve and the LP-EGRvalve may be modulated in addition to the back-pressure valve. Amodulation amplitude and frequency of the EGR valve's modulation may beadjusted based on the modulation (amplitude and frequency) of theback-pressure valve modulation, such that a desired EGR flow rate ismaintained. In one example, such as during conditions of no LP-EGR, theHP-EGR valve may be modulated in addition to the back-pressure valve tomaintain the EGR rate. In another example, such as during conditions ofno HP-EGR, the LP-EGR valve may be modulated in addition to theback-pressure valve to maintain the EGR rate. In another example, an EGRvalve may be selected based on response times. For example, the HP-EGRvalve may be selected if the HP-EGR valve moves faster (i.e. with ahigher frequency) in response to a request for modulation. In anotherexample, an EGR valve may be selected based on the degree of response.For example, the LP-EGR valve may be selected if a smaller amount ofmodulation is required for the LP-EGR valve (i.e. a smaller change inthe LP-EGR valve's position) as compared to a larger amount ofmodulation required for the HP-EGR valve (i.e. a larger change in theHP-EGR valve's position) to maintain the desired EGR rate. In stillanother example, the controller may select between the EGR valves basedon which valve is closer to a valve limit. For example, the controllermay select the LP-EGR valve for EGR valve modulation when the HP-EGRvalve is closer to a maximum or minimum limit (such as closer to a fullyopen or a fully closed position), or select the HP-EGR valve for EGRvalve modulation when the LP-EGR valve is closer to a maximum or minimumlimit (such as closer to a fully open or a fully closed position). Instill other examples, EGR valve modulation may include modulating boththe HP-EGR valve and the LP-EGR valve to maintain the EGR rate whilemodulating the back-pressure valve to provide exhaust gas mixing. Assuch, EGR valve modulation may include adjusting a modulation frequencyand amplitude of the selected EGR valve(s) to match the frequency of theback-pressure valve modulation, such that the effect of theback-pressure valve modulation on instantaneous EGR flow into the engineis offset. For example, as illustrated with reference to FIG. 4, the EGRvalve modulation may be at a common frequency, but out of phase with theback-pressure valve modulation to effectively cancel out theback-pressure valve modulation's effect on the instantaneous EGR flow.

Now turning to FIG. 4, example EGR and back-pressure valve operationsare illustrated on map 400 to further clarify the present disclosure.Map 400 depicts EGR valve adjustments without additional modulation at402, and with modulation at 412, and depicts back-pressure valve (hereinalso referred to as an exhaust throttle) adjustments without additionalback-pressure valve modulation at 404, and with modulation at 406.

Based on engine operating conditions, a desired amount of EGR may bedetermined by a controller. During a first EGR condition, when nosensing has been requested, the engine controller may adjust an EGRvalve (graph 402) and the back-pressure valve (graph 404) to provide therequested amount of EGR. Herein, the EGR valve may be the HP-EGR valveof the HP-EGR system and/or the LP-EGR valve of the LP-EGR system. Inone example, as depicted before t₁, the requested amount of EGR may beprovided by opening (or increasing an opening of) the EGR valve, andpartially closing (or decreasing an opening of) the exhaustback-pressure valve. Herein, during the first EGR condition, theadjusting of the valves is performed without modulation.

In comparison, during a second EGR condition, in response to a requestfor sensing received at t₁, the controller may adjust the EGR valve andthe back-pressure valve to provide the requested amount of EGR, and inaddition to the adjustment, the controller may modulate only theback-pressure valve at a higher frequency (graph 406) than the adjustingto promote exhaust gas mixing in the engine exhaust manifold. Followingthe modulation, one or more emission sensors may sense the mixed exhaustgas downstream of the back-pressure valve.

Further, during the second EGR condition, when modulating theback-pressure valve, the controller may determine a modulation frequencyand amplitude based on engine operating conditions. For example as shownat 408, during a first modulation between t₁ and t₂, the frequency ofthe back-pressure valve modulation may be decreased in response to adecrease in engine speed and an amplitude of the back-pressure valvemodulation may be increased in response to a valve timing moving awayfrom valve opening or closing. In another example, as shown at 410,during a second modulation between t₂ and t₃, the frequency of themodulation may be increased in response to an increase in engine speedand an amplitude of the modulation may be decreased in response to avalve timing moving towards the limit for valve opening or closing. Assuch, it will be appreciated that the modulation frequency of theback-pressure valve may always be maintained within the bandwidth of theback-pressure valve. Following sensing, after t₃, when no more sensingis requested, the back-pressure valve modulation may be stopped and theEGR valve adjustment and back-pressure valve adjustments may bemaintained.

During a third EGR condition, while the back-pressure valve is modulatedto generate a local turbulence for exhaust gas mixing, the EGR valve maybe modulated to maintain the EGR flow rate. Specifically, the EGR valvemay be modulated in coordination with the back-pressure valve modulationto enable exhaust gas mixing while maintaining the desired EGR flow. Asdiscussed earlier, the engine controller may select one or more of theLP-EGR valve and the HP-EGR valve for the EGR valve modulation. Forexample, as shown at 414, between t₂ and t₃, the EGR valve may bemodulated at the same frequency and amplitude as the correspondingback-pressure valve modulation, but with a phasing that may offset theback-pressure valve modulation.

In this way, an EGR system back-pressure valve may be modulated (e.g.,in addition to adjustments in response to providing the desired EGRamount) during EGR conditions to provide a local turbulence that maypromote exhaust gas mixing without affecting the average EGR flow. Bysensing mixed exhaust gas with sensors positioned downstream of themodulated valve, the accuracy of exhaust gas sensing may be improved,enabling more precise emission and EGR control. Further, by using EGRcomponents already present in the engine system for actively mixing theexhaust gas, the need for dedicated exhaust gas mixers may be reduced,thereby providing component reduction benefits.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

1-20. (canceled)
 21. A method for an engine including an EGR system,comprising, adjusting an EGR back-pressure valve below a first frequencyto provide a desired EGR amount; selectively modulating theback-pressure valve above the first frequency to promote exhaust gasmixing, while maintaining the desired EGR amount by modulating an EGRcontrol valve to cancel instantaneous EGR flow effects of theback-pressure valve modulation; and sensing the mixed exhaust gasdownstream of the back-pressure valve.
 22. The method of claim 21,wherein the modulation above the first frequency is in response to asensing request, and further wherein the modulation is in addition tothe adjusting.
 23. The method of claim 21, wherein the frequency ofmodulation is determined based on engine operating conditions includingan engine speed.
 24. The method of claim 23, wherein determining thefrequency of modulation includes increasing the frequency as enginespeed increases.
 25. The method of claim 23, wherein during sensing, anamplitude of the modulation is adjusted based on engine operatingconditions.
 26. The method of claim 25, wherein adjusting an amplitudeof the modulation includes, increasing an amplitude as engine valvetiming approaches a valve opening or closing limit.
 27. The method ofclaim 21, wherein the frequency of modulation is within a bandwidth ofthe back-pressure valve.
 28. The method of claim 21, further comprising,adjusting the EGR control valve to provide the desired amount of EGR.29. The method of claim 28, wherein the modulation of the EGR controlvalve is at a common frequency with the modulation of the EGRback-pressure valve.
 30. The method of claim 21, wherein a frequency ofthe EGR control valve modulation matches the frequency of theback-pressure valve modulation, the method further comprising adjustinga phase of the EGR control valve modulation to offset the back-pressurevalve modulation.
 31. The method of claim 21 wherein the EGR controlvalve is positioned in a high-pressure EGR system.
 32. A method for anengine including an EGR system, comprising, adjusting an EGRback-pressure valve downstream of a turbocharger, the adjusting below afirst frequency providing a desired EGR amount; and modulating theback-pressure valve above the first frequency to promote exhaust gasmixing, while maintaining the desired EGR amount by modulating alow-pressure EGR control valve to cancel EGR flow effects of theback-pressure valve modulation where modulation frequency increases withincreasing engine speed.
 33. The method of claim 34 further comprisingsensing the mixed exhaust gas downstream of the back-pressure valve. 34.The method of claim 33, wherein during sensing, an amplitude of themodulation is adjusted based on engine operating conditions.
 35. Themethod of claim 34, wherein adjusting an amplitude of the modulationincludes, increasing an amplitude as engine valve timing approaches avalve opening or closing limit.
 36. The method of claim 32, wherein themodulation of the low-pressure EGR control valve is at a commonfrequency with the modulation of the EGR back-pressure valve.
 37. Anengine system, comprising: an engine intake; an engine exhaust; anemission control device; a low-pressure EGR loop coupling the engineexhaust to the engine intake; a low-pressure EGR valve positioned in theEGR loop and configured to adjust an amount of exhaust gas divertedthrough the EGR loop; a back-pressure valve positioned downstream of theEGR loop and upstream of a tailpipe and within an exhaust flow leadingto atmosphere; one or more exhaust gas sensors positioned downstream ofthe back-pressure valve; and a control system with computer-readableinstructions for, adjusting the EGR valve and the back-pressure valve todivert an amount of exhaust gas through the EGR loop; and in response toa request for exhaust gas sensing, modulating the back-pressure valve ata frequency higher than the adjustment to promote exhaust gas mixingwhile maintaining the amount of exhaust gas diversion, and sensing themixed exhaust gas downstream of the back-pressure valve.
 38. The systemof claim 37 wherein the frequency of the modulation is adjusted based onengine operating conditions including engine speed.